EVALUATION OF THE VENTILATION RESISTANCE TO CONVECTIVE GAS-FLOW IN THE RHIZOMES OF NATURAL REED BEDS OF PHRAGMITES-AUSTRALIS (CAV) TRIN EX STEUD

The ventilation resistance of the internal gas-space system of common reed rhizomes (Phragmites australis (Cav.) Trin. ex Steud.) to convect ive gas through-flow was studied at reed stands of different lakes wit h respect to reed die-back. Some parameters for evaluating the contrib ution of the culms in ventilating the rhizomes are calculated. This in volves the culm's own pressure development (P-c), the counter-pressure within the rhizomes measured at the bottom of the culm (stubble; P-s) , and the resistance of the connected rhizome against gas-flow. The ad vantage of the relation between counter-pressure of rhizomes to the po tential pressure differential of the culms (P-s/P-c) and the ventilati on resistance parameter (VRP) is that they are not dependent on the ac tual pressurization due to changing weather conditions. A comparison o f different sites is possible even when measured at different times. M ost of the reed stands so far investigated show low resistances to con vective gas-flow during summer but an increase in resistance was measu red during the later vegetation season. Low numbers of old culms funct ioning as efflux culms can at least partly be compensated by the funct ional change of living shoots to efflux culms. The gas composition of these green shoots and the dead efflux culms is lower in oxygen and hi gher in carbon dioxide than influx culms, and additionally methane is present. Very high resistances against gas through-flow could only be detected at sites where the formerly homogeneous distribution of the s hoots was changed to a cluster-like distribution with only few or no o ld culms.